1. Field of the Invention
The present invention relates to an electrochromic display device.
2. Description of the Related Art
In recent years, development of electronic paper is actively taking place as an alternative electronic medium to paper. Electronic paper has a feature of using a display device as a paper, and therefore electronic paper is demanded to have features different from conventional display devices such as CRT and a liquid crystal display.
For example, electronic paper is demanded to have features such as being a reflective display device with a high white reflection ratio and a high contrast ratio, being able to display images with high precision, having a memory effect for the display, capable of being driven at low voltage, being thin and light, and being low-cost. Among these, there is particularly high demand to have the same white reflection ratio and the same contrast ratio as those of paper, as features relevant to the quality of the display.
In the past, as display devices using electronic paper, for example, a method of using a reflective liquid crystal, a method of using electrophoresis, and a method of using toner migration, have been proposed. However, with any of these methods, it has been very difficult to perform multicolor display while securing a white reflection ratio and a contrast ratio.
Generally, in order to perform multicolor display, a color filter is provided. However, when a color filter is provided, the color filter itself absorbs light, and the reflection ratio decreases. Furthermore, in a color filter, each pixel is divided into three colors of red (B), green (G), and blue (B), and therefore the reflection ratio of the display device decreases, which leads to a decrease in the contrast ratio. When the white reflection ratio and the contrast ratio decrease significantly, the legibility is drastically degraded, and therefore it is difficult to properly use the electronic paper.
Meanwhile, as a promising method for realizing a reflective display device without providing a color filter as described above, there is a method of using an electrochromic phenomenon.
A phenomenon in which voltage is applied to cause a reversible oxidation-reduction reaction and to reversibly change the color is referred to as electrochromism. An electrochromic display device uses the color-forming and color-erasing effects (hereinafter, “color forming/erasing”) of an electrochromic compound which induces the electrochromism phenomenon. This electrochromic display device is capable of expressing colors by absorption of light, and has a memory effect. Therefore, research and development is taking place extensively as a promising candidate of the display device technology applicable to a reflective display using electronic paper, photochromic glass, photochromic lenses, etc., ranging from the development of materials to the design of the device.
However, the electrochromic display device has a shortcoming in that the response speed of color forming/erasing is slow, due to the principle that color forming/erasing is performed by using the oxidation-reduction reaction. Patent Document 1 discloses an example of an electrochromic display device in which the response speed of color forming g/erasing is improved by fixing the electrochromic compound near the electrode. According to the description in Patent Document 1, the time required for color forming/erasing has been approximately 10 seconds in the conventional technology, but the time is reduced to approximately one second, both in terms of the time taken to form a blue color from a clear color and the time taken to erase the blue color to form a clear color. However, this is not sufficient. In the research and development of electrochromic display devices, there is a need to further increase the response speed of color forming/erasing.
Meanwhile, in an electrochromic display device, various colors can be formed by the structure of the electrochromic compound, and therefore there are high expectations to use the electrochromic display device as a multicolor display device.
There are several known examples of multicolor display devices using such an electrochromic display device. For example, Patent Document 2 discloses a multicolor display device using an electrochromic compound, in which particles of plural kinds of electrochromic compounds are laminated. Patent Document 2 discloses an example of a multicolor display device in which a multicolor display electrochromic compound is realized by laminating plural layers of electrochromic compounds which are highly-polymerized compounds having plural functional groups of different voltages that implement color forming.
Furthermore, Patent Document 3 discloses a display device in which multiple electrochromic layers are formed on an electrode, and multiple colors are formed by using the difference in voltage values and current values required for forming the respective colors. Patent Document 3 discloses an example of a multicolor display device which forms different colors, and which includes display layers formed by laminating or mixing plural electrochromic compounds having different threshold voltages for forming colors and different requisite electrical charge amounts required for forming colors.
Furthermore, Patent Document 4 discloses an example of a multicolor display device including a lamination of plural structure units. Each structure unit includes an electrochromic layer and an electrolyte being sandwiched between a pair of transparent electrodes. Furthermore, Patent Document 5 discloses an example of a multicolor display device in which a passive matrix panel and an active matrix panel are constituted by using the structure units described in Patent Document 4 to form the three colors of RGB.
Furthermore, Patent Document 6 discloses an example of an electrochromic display device, in which plural display electrodes are provided so as to be isolated from each other between a display substrate and an opposing electrode, plural electrochromic layers are provided in accordance with the plural display electrodes, and the electric resistance between one display electrode and another display electrode is made higher than the electric resistance of the one display electrode, to form a desired color.
However, the conventional electrochromic display device has the following problems.
For example, an electrochromic display device performs color forming/erasing by causing oxidization or a reductive reaction with electric charges supplied from the outside. Therefore, when a reductive color forming is caused for a certain pattern, the electrochromism material is in a reductive state according to the pattern. When erasing this pattern, the electrochromic display device is driven so as to cause oxidization color erasing with the same pattern used for forming the color, to erase the pattern that has been displayed. The same applies to the case of oxidization color forming and reduction color erasing. Furthermore, the color is formed in a stable state, and by supplying electric charges from the outside, the color is erased or a different color is formed.
The electrochromic display device has the above properties. Therefore, as the number of times of rewriting the pattern increases, the amount of electric charges supplied from the power source increases. Therefore, for example, when the electrochromic display device is used as a rewritable display device for reproducing a video, it is important to further reduce the consumption amount of electric charges when rewriting patterns.    Patent Document 1: Japanese National Publication of International Patent Application No. 2001-510590    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-121883    Patent Document 3: Japanese Laid-Open Patent Publication No. 2006-106669    Patent Document 4: Japanese Laid-Open Patent Publication No. 2003-270671    Patent Document 5: Japanese Laid-Open Patent Publication No. 2004-151265    Patent Document 6: Japanese Laid-Open Patent Publication No. 2000-292818